Titanium diboride (TiB2) is a very chemically stable intermetallic compound formed between titanium and boron. Titanium diboride is extremely hard, nearly as hard as diamond, which makes it useful as a cutting tool. It also has good wear resistance and so is a good candidate for use in wear parts, wear resistant coatings, and seals. Titanium diboride is also tough enough to be used in some ballistic armor applications. It resists oxidation and does not react with many molten, nonferrous metals, including copper, zinc, and aluminum. Titanium diboride is an excellent conductor of both heat and electricity. Its chemical stability, conductivity, and high melting point make it useful in many high temperature manufacturing applications, for example, thermocouple protection tubes, crucibles for handling molten metals, boats for carrying materials through furnace hot zones, and vacuum metallizating process components. Titanium diboride is also used as an electrode material for electrolytic refining, or electrowinning, of some nonferrous metals, e.g., aluminum.
Titanium diboride does not occur naturally. Bulk titanium boride is produced as a powder that is then consolidated together by the application of pressure and high temperature. There are several conventional processes for synthesizing titanium diboride powder, for example: mechanically alloying together elemental powders of titanium and boron; carbothermally reducing titanium dioxide (TiO2) in a reaction with one or more of boron oxide (B2O3), boron carbide (B4C), titanium carbide (TiC), and carbon (C); solid state reacting titanium tetrachloride (Ti4Cl), magnesium (Mg), and magnesium diboride (MgB2); arc plasma reacting gaseous titanium tetrachloride (Ti4Cl), boron trichloride (BCl3), and hydrogen (H2); and self-propagating high-temperature synthesizing from titanium dioxide (TiO2), boron oxide (B2O3), and magnesium (Mg). These processes may include ancillary purifying steps to rid the titanium diboride powder of contaminants from the powder making process. They may also include mechanical milling steps to reduce the powder particle size and/or to deagglomerate the powder.
While submicron titanium diboride powders have relatively good sinterability due their high surface area-to-volume ratios, they are difficult and expensive to use because special handling precautions must be employed on account of their high pyrophoricity and propensity to oxidize. Coarser titanium diboride powders are relatively difficult to sinter because of several factors: the largely covalent nature of titanium diboride's chemical bonds, titanium diboride's low self-diffusion rates, and the formation of boron oxide on the powder surfaces. Thus, it is usually necessary to use expensive high temperature processes such as hot pressing and hot isostatic pressing to consolidate titanium diboride powders at high temperatures and pressures into blocks or other shapes. It is often necessary to machine the consolidated titanium diboride to achieve a desired useful component. The high hardness of titanium diboride makes the machining expensive.
The less-expensive ceramic powder processing methods of cold pressing a powder-plus-volatile binder mixture to shape followed by binder removal and furnace sintering to achieve a net or near-net shape do not work well with titanium diboride unless sinter aid materials are added to the powder. Sintering aids typically lower the sintering temperature while promoting densification. The lower sintering temperatures also help to avoid grain growth of the titanium diboride grains during the sintering furnace treatments. Known sintering aids for titanium diboride include iron (Fe), cobalt (Co), nickel (Ni), and chromium (Cr), all of which form relatively low-melting temperature eutectics with boron. Carbon (C) is another known sinter aid material, whose beneficial effects are thought to be due to its ability to reduce the boron oxide (B2O3) that coats the titanium diboride particle surfaces. Other known sinter aid materials for titanium diboride include several borides, e.g., chromium diboride (CrB2) and nickel boride (NiB), and boron nitride (BN). It is generally necessary to use about 1-10 weight percent sintering aid to be effective.
Sintering aids may also be used to lower the temperatures and shorten the hold times for hot pressing or hot isostatic pressing temperature titanium diboride.
Conventional sintering aids, however, have their draw backs. With the exception of carbon, all of the sinter aids identified above rely on causing the formation of grain boundary phases for their effectiveness, i.e., they result in the formation of additional phases at the grain boundaries of the titanium diboride grains. These grain boundary phases may lower the strength and toughness of the titanium diboride article. They may also make the titanium diboride article more susceptible to corrosive failure due to chemical attack at its grain boundaries, especially in extremely hostile, corrosive applications such as the use of titanium diboride article as a cathode in the electrowinning of aluminum.
What is needed is a new type of sinter aid for titanium diboride that does not result in the formation of grain boundary phases in the sintered titanium boride article.
Research by others has suggested that a small addition of tungsten carbide (WC) and cobalt may be used as sinter aid for another boride, i.e., zirconium diboride (ZrB2), as disclosed by A. L. Chamberlain et al., “Pressureless Sintering of Zirconium Diboride,” Journal of the American Ceramics Society, Vol. 89, Issue 2 (2006), pages 450-456. Those researchers speculate that a complex chemical reaction occurs during the sintering heat treatment cycle which results in some of the carbon from the tungsten carbide eliminating the boron oxide from the zirconium diboride particle surface and some of it forming zirconium carbide (ZrC). They also speculate that the tungsten (W) and zirconium carbide form a solid solution with the titanium diboride. The joint effect of these occurrences is to improve the sinterability of the cold pressed and sintered zirconium diboride powder. In addition to these suggested mechanisms, it is likely that the cobalt portion of the tungsten carbide-cobalt addition is acting in its conventional sinter aid role to improve the sinterability of the zirconium diboride.